F-SECURE INTERNET GATEKEEPER FOR LINUX 4.01 Administrator's Manual page 44

Transparent proxy
Transparent Proxy mode (transparent)
Enables transparent proxy mode.
A NAT redirection setting is required when the proxy operates as a transparent proxy. Use one
of the following methods to specify the NAT redirection setting:
• Use the " Edit NAT (iptables) redirect settings". To do this, click
redirect settings.
• Use the iptables command from the command line to specify the setting as follows. (The
example shows the port number being set to 9025.)
# iptables -t nat -A PREROUTING -i eth1 -p tcp --dport 25 ¥
-j REDIRECT --to-port 9025
For more information, see
Edit NAT (iptables) redirect settings
NAT
Specifies the NAT redirection settings. If you select the
connections for port 25 are redirected to the SMTP proxy (port 9025).
F-Secure Internet Gatekeeper for Linux/Administrator's Guide
"Transparent Proxy", 108.
Edit NAT (iptables)
checkbox, all
SMTP redirect
44